1. Field of the Invention
The present invention generally relates to a semiconductor sensor using a piezoresistor such as a semiconductor acceleration sensor or a semiconductor angular velocity sensor and a manufacturing method of the same. More specifically, the present invention relates to a semiconductor sensor and a manufacturing method of the same in which the semiconductor sensor includes a weight part, a supporting part formed around and separated from the weight part, a flexible part connected between the weight part and the supporting part so as to support the weight part, a semiconductor layer made of semiconductor materials in at least one part of each of the weight part, the supporting part, and the flexible part, and plural piezoresistors formed in the semiconductor layer of the flexible part.
Such a semiconductor sensor, for example, is used for measuring acceleration of a moving vehicle in the vehicle longitudinal direction or in the vehicle width direction or degree of jiggling of a hand upon using a video camera.
It should be noted that the term “semiconductor substrate” described in the claims of the present invention and this specification includes not only a substrate made of only semiconductor materials but also an SOI (Silicon-on-Insulator) substrate including an insulating film formed therein.
2. Description of the Related Art
As a semiconductor sensor, an acceleration sensor used in a vehicle is known. For example, there is an acceleration detecting device using an piezoresistive device as shown in FIGS. 13A though 13D (see Japanese Patent No. H8-7228). As shown in FIGS. 13A though 13D, the acceleration detecting device is a flat and compact semiconductor sensor 71 having a size of approximately 3 mm by 2 mm. The semiconductor sensor 71 includes a weight part, a flexible part, and a supporting part. The weight part, the flexible part, and the supporting part are integrally formed by etching silicon using a potassium hydrate aqueous solution. In the flexible part of the acceleration detecting device, there is provided a piezoresistive device whose resistance is changed in accordance with the bending of the flexible part due to the displacement of the weight part caused by acceleration, thereby detecting the change of resistance of the piezoresistive device as the acceleration.
FIGS. 13A though 13D are a perspective view, a plan view, cross-sectional views taken along lines A-A′ and B-B′ of FIG. 13B, respectively, showing an example of a conventional semiconductor sensor 71.
As shown in FIGS. 13A though 13D, the semiconductor sensor 71 is formed using an SOI substrate including a first semiconductor layer 5, a second semiconductor layer 9, and an insulating layer 7 sandwiched between the first semiconductor layer 5 and the second semiconductor layer 9. From a different point of view, the semiconductor sensor 71 includes a frame-shaped supporting part 11 made of the SOI substrate 3, and flexible parts 73 each made of the first semiconductor layer 5 and connected to the supporting part 11. There are plural piezoresistive elements 19 formed in the first semiconductor layer 5 of the flexible part 73. In the center side of the supporting part 11, there is provided a weight part 75 surrounded by and separated from the supporting part 11. The weight part 75 includes the first semiconductor layer 5, the insulating layer 7, and the second semiconductor layer 9. The first semiconductor layer 5 of the weight part 75 is continuously formed with the first semiconductor layer 5 of the flexible part 73. Because of this structure, the weight part 75 is supported by the flexible part 75.
On a first surface 3a of the SOI substrate 3, an insulating film 21 is formed. In FIGS. 13A and 13B, the piezoresistive elements 19 are shown for illustrative purposes. On the insulating layer 21, plural metal wiring patterns 23 and plural pad electrodes 25 are formed. The metal wiring patterns 23 are electrically connected to the corresponding piezoresistive elements 19 via through holes formed in the insulating film 21.
A protection film 27 is formed on the insulating film 21 so as to cover not only the insulating film 21 but also areas where the metal wiring patterns 23 are formed on the insulating film 21. An opening is formed in the protection film 27 on each pad electrode 25. The protection film 27 is not shown in FIGS. 13A and 13B for illustrative purposes only.
A second surface 3b (opposite to the first surface 3a) of the supporting part 11 and a glass substrate 29 are bonded together by anodic bonding. As a result, the surface of the weight part 75 on the second surface 3b of the SOI substrate 3 side is separated from the glass substrate 29.
FIGS. 14A though 14F are cross-sectional views taken along line A-A′ in FIG. 13B, illustrating steps of a manufacturing method of the semiconductor sensor 71. Each of the parenthetical numbers shown in FIGS. 14A through 14F corresponds to the step of the manufacturing method described below. Next, a manufacturing method of a conventional semiconductor sensor is briefly described with reference to FIGS. 13A through 14F.
Step (1): As shown in FIG. 14A, a thermal oxide film 69 is formed on the second surface 3b of the SOI substrate 3 including the first semiconductor layer 5, the insulating layer 7, and the second semiconductor layer 9. The piezoresistive elements 19 are formed in the vicinity of the surface of the first semiconductor layer 5 on the first surface 3a side of the SOI substrate 3. The insulating film 21 is formed on the first surface 3a of the first semiconductor layer 5. Through holes are formed at prescribed positions in the insulating film 21. The metal wiring patterns 23 and the pad electrodes 25 are formed on the area of the insulating film 21 including the areas where the through holes are formed (see FIG. 13C). A protection film 27 is formed on the surface of the insulating film 21. An opening (not shown) is formed in the protection film 27 and on each of the pad electrodes 25.
Step (2): As shown in FIG. 14B, by photoengraving and etching techniques, the thermal oxide film 69 on areas where the flexible parts 73 and the weight part 75 are to be formed excluding at least an area where the supporting part 11 is to be formed is selectively removed.
Step (3): As shown in FIG. 14C, by a photoengraving technique, the resist pattern 77 is formed on the second surface 3b of the SOI substrate 3 so that the resist pattern 77 covers the area where the supporting part 11 and the weight part 75 are to be formed and an opening is formed on the area where the flexible parts 73 are to be formed. Then, by an etching technique, the second semiconductor layer 9 on the area where the flexible parts 73 are to be formed is selectively removed by using the resist pattern 77 as a mask.
Step (4): As shown in FIG. 14D, after the resist pattern 77 is removed, the second semiconductor layer 9 on the second surface 3b side of the SOI substrate 3 in the area where the weight part 75 is to be formed is etched. Namely, the thickness of the second semiconductor layer 9 in the area where the weight part 75 is to be formed is reduced to form the weight part 75. A resist pattern (not shown) for defining the area where the flexible parts 73 and the weight part 75 are to be formed is formed on the second surface 3b side of the SOI substrate. By using the resist pattern, the insulating layer 7, the first semiconductor layer 5, insulating film 21, and the protection film 27 in the area other than the areas where the flexible parts 73 or the weight part 75 is to be formed inside the area where the supporting part 11 is to be formed are removed by an etching technique to form the flexible parts 73 and the weight part 75.
Step (5): As shown in FIG. 14E, the thermal oxide film 69 is removed. In this step, the insulating film 7 in the area where the flexible parts 73 are to be formed is also removed to form the flexible parts 73 made of the first semiconductor layer 5.
Step (6): As shown in FIG. 14F, a stopper substrate 29 and a surface of the second semiconductor layer 9 on the second surface 3b side of the SOI substrate 3 in area including the area where the supporting part 11 is to be formed are bonded together by, for example, anodic bonding.
Step (7): Finally, each of the semiconductor sensors 71 is cut off from the SOI substrate 3 to complete the manufacturing steps of the semiconductor sensor 71 (see FIGS. 13A through 13D).
In the description above, the semiconductor sensor 71 is formed using the SOI substrate 3 as a semiconductor substrate. However, the present invention is not limited to the semiconductor sensor formed from the SOI substrate. For example, the semiconductor sensor may be formed of a semiconductor substrate including semiconductor materials only (see Japanese Patent Application Publication No. 2003-270262).
Further, as disclosed in the Japanese Patent Application Publication No. 2003-270262, in the semiconductor sensor, there may be plural flexible parts formed so that each of the flexible parts connects between one of the surfaces of the weight part facing the supporting part in plan view and the supporting part (double holding type) or there may be a single flexible part connected between one surface of the weight part and the supporting part (single holding type).
Further, recently, there has been a growing demand for reducing the size and the thickness of chips. To respond to the demand, a semiconductor sensor having a weight part that is made of a metal material having a specific gravity greater than that of a semiconductor material is disclosed (see, for example, Japanese Patent Application Publication No. 2006-250653). By forming the weight part made of a material having a specific gravity greater than that of a semiconductor material, the weight of the weight part increases compared with a weight part made of silicon and having the same size as that of the weight part made of the metal material, thereby reducing the size and thickness and improving the sensitivity of the semiconductor sensor.
Still further, to increase the volume of the weight part, a semiconductor sensor including a weight part having a plan-view shape different from a rectangular shape such as a cloverleaf shape is disclosed (see for example, Japanese Patent Application Publication No. 2007-033355).
To improve the sensitivity of a semiconductor sensor in which the weight part, the flexible part, and the supporting part are integrally formed of silicon, there is a method for reducing the width or thickness, or increasing the length of the flexible part. However, in this method for improving the sensitivity, there may be a problem in that the mechanical strength of the beam part (flexible part) becomes not strong enough to withstand the stresses during the manufacturing process. As another method to improve the sensitivity, there is a method for increasing the weight of the weight part. However, it is necessary to increase the size of the weight part so as to increase the weight of the weight part, which goes against the demand for reducing the size of semiconductor sensors.
Further, unfortunately, in a semiconductor sensor including a weight part made of a metal material to have a specific gravity greater than that made of a semiconductor material, there is a problem in that such a weight part made of a metal material may not be formed in a typical manufacturing process of a semiconductor device, thereby causing an increase of the number of manufacturing steps and accordingly the manufacturing cost.